This invention relates to fusion welding at constant temperature with predetermined minimum temperature rise times.
Modern repair and manufacturing fabrication methods often require welding. Unlike other joining methods, welding makes joints which are as strong as, or almost as strong as, the basic materials. Primitive welding methods involved the use of a brazier of coals, an anvil and a hammer. Welds made by use of such equipment might be good or bad, depending upon the skill of the craftsman using them, but would certainly be variable.
The increasing stress upon reliability has led to automated welding arrangements and to sophisticated electrical welders, such as that described in U.S. Pat. No. 4,359,622 issued Nov. 16, 1982 to Dostoomian et al. The Dostoomian arrangement includes a complex controller and different types of temperature sensors for applying electrical power to the work piece to cause the temperature of the work piece in the region being welded to follow the temperature profile of a previously welded piece which tests have shown to have sufficient strength.
It may be desirable for many reasons to have even more reliable welds. For example, automobile manufacturers have economic incentives (such as warranty costs) to produce welds which are highly consistent and therefore have well defined strength characteristics from one weld to the next, and which further have a microstructure which is not conducive to corrosion or to failure due to the propagation of microcracks attributable to stress-related corrosion.
During the formation of a weld, intricate fusion occurs between the materials of the two pieces to be joined. For example, if separate pieces of silver (Ag) and copper (Cu) are to be welded, then Ag atoms diffuse into the Cu, and Cu atoms diffuse into the Ag during the formation of the weld. Statistical theory, described for example in the text "Diffusion In Solids" by P. G. Shewmon, published by McGraw Hill Book Company, New York, 1963 can describe the movement of an atom from its original position in a particular time t at a particular temperature T. The expression is given by: EQU x=(6Dt).sup.1/2 ( 1)
where x is the distance from the original position, D is a diffusion coefficient at a temperature T, and t is time. Diffusion coefficient D, in turn, is given by the relationship: EQU D=D.sub.o exp(-Q/kT) (2)
where D.sub.o is a pre-exponential diffusion factor, Q is an activation energy of diffusion, T is temperature in degrees Kelvin, and k is Boltzmann's constant (1.99 cal/mole K).
Parameters D.sub.o and Q are parameters which are defined for particular materials. These values have been measured experimentally and the data is available in the published literature, as in for example the "Handbook of Chemistry and Physics", CRC Press, Cleveland, Ohio, 1974. From the equations and constants, it is clear that for a particular time duration of the weld (welding time), an atom moves further from its original position at higher welding temperatures. If the welding temperature is constant, an atom moves further from its original position if it is given a longer time in which to move, i.e., at longer weld durations.
In almost any ordinary welding, atoms of different types are involved. This is true, even if the pieces being joined are nominally of the same material. This is because the materials are never absolutely pure, but ordinarily contain substantial impurities. For example, silver is available in "coin silver" form, which is 90% pure, with the principal portion of the remainder being copper; and with purities such as 99.5% and 99.9% the impurities being carbon, nickel, sulfur, oxygen and chlorine. It is well known that iron often includes carbon and small amounts of many other elements such as chromium, manganese, nickel, silicon, and titanium. During welding of dissimilar materials or even of similar materials, different compounds or different solid phases of the same compound can be formed within the heated region. For convenience, these are all referred to as phases. In general, such solid phases are areas of the solid which have different chemical or microstructural composition than the bulk material. The strength of a welded joint can depend upon the number of inclusions of phases other than the main or desired phase, and also upon the size of the included phases.
There are a number of theories which address the nucleation and growth of new phases. Exact expressions for nucleation and growth rates are complex, and depend upon a number of different variables. The dependence of the nucleation of a new phase in terms of temperature and time can be expressed generally by the equation: EQU I.alpha.exp(-G/kT) (3)
where I is a nucleation rate, G is an activation energy, k is Boltzmann's constant and T is temperature. Equation 3 makes it clear that the nucleation rate of a new phase increases exponentially with an increase in temperature.
Once nuclei of a new phase are formed, during the initial stages, the growth rate at particular temperature is expressed by: EQU Y.alpha.(t).sup.3/2 ( 4)
where Y is a growth rate, and t is time. Equation 4 demonstrates that nuclei of a new phase grow, at least during the initial stages, at a rate proportional to t.sup.3/2. Thus, at longer weld durations, larger size particles are present in the weld zone compared with the particles formed at shorter weld times. The size of the different phases included in the weld zone can strongly affect the characteristics of the weld. Naturally, it is desirable that the weld have virtually no nucleated phases other than the desired phase of the bulk material, or that if such phases occur, that the size of the inclusion be small.
As a particular example, if two pieces of silver, each including 0.5% nickel impurities, are welded, an undesired phase consisting of Ag.sub.3 Ni may nucleate and grow in roughly spherical form. The nucleated Ag.sub.3 Ni is 25% Ni instead of 0.5%, which depletes the bulk material of Ni. The amount of Ni in the bulk of the weld, therefore, will depend upon the number of nucleation sites, and upon the length of time and rate of growth of the nucleated solid phases. Other phases may have elongated forms or crystalline structure. Depending upon the size and physical characteristics of the interface between the nucleated phase region and the bulk material, the weld region may be more or less brittle, have different strength and may even have different characteristics in various directions (anisotropy). A welding technique is desired which provides improved weld-to-weld consistency.